Is it possible to bulk reclassify a document in Vault?
No. Currently, it is possible to reclassify the latest version of a document and only one document at a time. Vault Loader and the Bulk API do not currently support Bulk Reclassify.
A single document can only be reclassified through the UI (User Interface) or through the regular API.
Vault API Documentation: API: Reclassify Document
Vault Reclassify Documentation: Reclassifying Documents